Field of the Invention
The invention relates to a method for etching phase shift layers of half-tone phase masks.
The trend in microchips is toward an ever greater integration density of the electronic components. This increases the storage density of memory chips and also lowers the price of the individual component. The fabrication of microchips includes lithographic steps, in which a mask is used to reproduce the structure, which is to be produced on a substrate, in a photosensitive resist layer. After the resist layer has been developed, the result is a resist structure, which in turn can be used as a mask for etching processes or during the deposition of patterned semiconductor layers. A problem in this context is primarily the reproduction of the edges of the resist structure which is subsequently obtained. In this respect, it is necessary for the transition from the exposed region to the unexposed region to be as sharp as possible. As the minimum feature size of the components decreases, the relative size of the blurred region in the transition from the exposed region to the unexposed region increases in relation to the size of the component. In order to obtain a sharp contrast between exposed and unexposed regions in the photoresist even for very small structures, half-tone phase masks have been developed. A patterned layer of a phase shift material is deposited on a substrate which is transparent to the exposure wavelength. While the exposure radiation passes through the substrate without undergoing a phase shift in the regions which are not covered by the phase shift material, the exposure radiation undergoes a phase shift of 180xc2x0 in the region which is covered by the phase shift material. The transmission of the exposure radiation is low in the region which is exposed with the phase shift material and is only approximately 3 to 10%. At the boundary between the uncovered regions and the regions which are covered with the phase shift material, the radiation in the reproduction or image formation on the photoresist has a phase difference of 180xc2x0. Therefore, in this region it is extinguished, resulting in a very sharply delineated transition in the photoresist between the exposed regions and the unexposed regions, and therefore also in a sharp edge form.
To fabricate half-tone phase masks, a layer of the phase shift material is patterned through the use of plasma etching using a mask. It is of crucial importance for the phase shift properties of the mask that, during etching of the phase shift layer, the substrate below it should not be etched or should only be etched to the minimum possible degree. This requires a high selectivity of the plasma-etching process, i.e. during the etching only the layer of the phase shift material should be etched, while the substrate provided below it should not be etched. Furthermore, as the minimum feature sizes become ever smaller, there should be no loss of dimensions. This means that there should be no undercut etching of the phase shift layer below the masking layer.
Hitherto, it has only been possible either to achieve a high etching selectivity or to avoid a loss of dimensions. C. Constantine, D. Johnson, R. J. Westerman, and A. Hourd (SPIE Proc., Vol. 3546, 1998, p. 88-97) have investigated the etching operation as a function of a change in various parameters for the conventional etching plasmas. The plasmas which are customary for industrial applications are produced from CHF3/O2, CF4/O2 and SF6. A high selectivity of more than 3:1 can be achieved by the use of low-polymer etching chemicals, such as SF6, with a low cathode power of the etching reactor. However, these etching processes predominantly take place isotropically and therefore result in a pronounced undercut etching of the mask and therefore in a considerable loss of dimensions. Undercut etching is avoided by the use of polymer-rich etching chemicals, such as for example CHF3/O2 or CF4/O2, at a higher cathode power. However, these processes have a poor etching selectivity with respect to an SiO2 substrate, making the process difficult to control. In the event of excessive removal of material from the substrate by etching, the resulting phase angle of the substrate/phase shift material transition exceeds the required tolerances. As a result, the reproduction quality of the half-tone phase mask deteriorates, and it is no longer possible for the smallest possible structures to be reproduced without defects.
It is accordingly an object of the invention to provide a method for etching phase shift layers of half-tone phase masks which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which allows phase masks with a high reproduction quality to be fabricated.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for etching phase shift layers in half-tone phase masks, the method includes the steps of:
depositing a phase shift layer on a substrate;
applying a mask to the phase shift layer, the mask having a structure to be transferred into the phase shift layer; and
etching the structure into the phase shift layer by carrying out a plasma etching in an etching reactor with a plasma produced from a fluorinated hydrocarbon having a ratio F/Hxe2x89xa62 and O2.
According to another mode of the invention, CH3F is used as the fluorinated hydrocarbon.
According to yet another mode of the invention, the cathode power for the etching reactor is selected to be greater than 20W.
According to a further mode of the invention, the molar ratio of the fluorinated hydrocarbon to the O2 is selected to be between 2:1 and 1:2 for producing the plasma.
According to another mode of the invention, an inert gas is added to the plasma for etching the structure into the phase shift layer.
According to yet another mode of the invention, the phase shifter layer is composed of a molybdenum/silicon nitride (MoSiN).
According to another mode of the invention, the substrate is composed of quartz glass.
According to a further mode of the invention, the mask is configured as a chrome mask.
According to an additional mode of the invention, a re-etching step is carried out subsequent to etching the structure into the phase shift layer.
According to another mode of the invention, a plasma produced from SF6 is used for carrying out the re-etching step.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is described herein as embodied in a method for etching phase shift layers of half-tone phase masks it is nevertheless not intended to be limited to the details described, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments.